1. Field of the Invention
The present invention relates to a method and an apparatus for monitoring the dynamics of gas sensors of an internal combustion engine which are disposed, for example, as gas probes in the exhaust gas duct of an internal combustion engine as part of an exhaust gas monitoring and abatement system or as gas concentration sensors in an intake air passage of the internal combustion engine, the gas sensor exhibiting a low-pass behavior as a function of geometry, measurement principle, aging, or contamination, a dynamics diagnosis being carried out, upon a change in the gas state variable to be measured, on the basis of a comparison between a modeled and a measured signal, and the measured signal being an actual value of an output signal of the gas sensor and the modeled signal being a model value.
2. Description of the Related Art
In order to reduce emissions in passenger cars having Otto-cycle engines it is usual to use, as exhaust emission control systems, three-way catalytic converters that convert exhaust gas adequately only when the air/fuel ratio λ is regulated with high precision. For that purpose, the air/fuel ratio λ is measured using an exhaust gas probe upstream from the exhaust emission control system. The ability of an exhaust emission control system of this kind to store oxygen is exploited in order to accept oxygen in lean phases and emit it again in rich phases. The result of this is that oxidizable pollutant gas components of the exhaust gas can be converted. An exhaust gas probe downstream from the exhaust emission control system serves to monitor the oxygen storage capacity of the exhaust emission control system. The oxygen storage capacity must be monitored in the context of onboard diagnosis (OBD), since it represents an indication of the conversion capacity of the exhaust emission control system. In order to determine the oxygen storage capacity, either the exhaust emission control system is firstly loaded with oxygen in a lean phase and then purged in a rich phase with a known lambda value in the exhaust gas in consideration of the quantity of exhaust gas passing through, or the exhaust emission control system is firstly purged of oxygen in a rich phase and then filled in a lean phase with a known lambda value in the exhaust gas in consideration of the quantity of exhaust gas passing through. The lean phase is terminated when the exhaust gas probe downstream from the exhaust emission control system detects the oxygen that can no longer be stored by the exhaust emission control system. Likewise, a rich phase is terminated when the exhaust gas probe detects the passage of rich exhaust gas. The oxygen storage capacity of the exhaust emission control system corresponds to the quantity of reducing agent delivered for purging during the rich phase, or to the quantity of oxygen delivered for filling during the lean phase. The exact quantities are calculated from the signal of the upstream exhaust gas probe and from the exhaust gas mass flow ascertained from other signals.
If the dynamics of the upstream gas probe decrease, for example due to contamination or aging, the air/fuel ratio can then no longer be regulated with the necessary precision, so that the conversion performance of the exhaust emission control system declines. Deviations can also occur in the diagnosis of the exhaust emission control system, and can cause an exhaust emission control system that is in fact operating correctly to be wrongly evaluated as non-functional. Legislation requires a diagnosis of probe properties during driving operation in order to ensure that the required air/fuel ratio can continue to be established with sufficient accuracy, that emissions do not exceed permissible limits, and that the exhaust emission control system is being correctly monitored. OBDII provisions require that lambda probes and other exhaust gas probes be monitored not only in terms of their electrical functionality, but also with regard to their response behavior; in other words, a deterioration in probe dynamics, which can become evident due to an increased time constant and/or a dead time, must be detected. Dead times and delay times between a change in exhaust gas composition and the detection thereof must be tested onboard as to whether they are still permissible for user functions, i.e. for control, regulation, and monitoring functions that utilize the probe signal. The dead time from a change in mixture until the signal edge, and a specific rise time, e.g. from 0% to 63% or from 30% to 60% of a signal swing, are typically used as parameters for the dynamic properties of exhaust gas sensors. The dead time also encompasses the gas transit time from the engine outlet to the probe, and therefore changes in particular when the sensor installation location is manipulated.
The gas sensors or gas concentration sensors used in diesel engines are broadband lambda probes and, in connection with SCR catalytic converters, also NOx sensors. The latter also additionally supply an O2 signal. The O2 signal of a broadband lambda probe or NOx sensor is used in a diesel engine not only for the operation of exhaust gas post-processing devices, but also for emissions reduction within the engine. The measured O2 concentration in the exhaust gas, or the measured lambda signal, is used to establish the air/fuel mixture accurately in dynamic fashion, and thus to minimize the variability of the raw emissions. In diesel engines having an NOx storage catalytic converter (NSC), a broadband lambda probe is required respectively before and after the catalytic converter for reliable representation of the rich mode for regeneration. Emissions reduction inside the engine and NSC operation likewise impose certain minimum requirements in terms of the dynamic properties of the O2 probe. Nowadays the rise time of the O2 signal is monitored at the transition from load to coast, i.e. upon an increase from a specific percentage below the normal O2 content of air to 21%. If the sensor signal has not reached a specific intermediate value after a maximum time, this is interpreted as a dead-time fault. In diesel engines having an NOx storage catalytic converter (NSC), the response behavior of the lambda probes before and after the catalytic converter is also usually compared.
It is to be expected, for future vehicle generations and model years, that monitoring of the sensor dynamics in a context of decreasing O2 concentration will also be required. In addition, with hybrid vehicles there will in the future be no coasting phases and therefore no phases with a constant O2 concentration of 21%.
Initial approaches to solutions to these additional requirements are active monitoring in published German patent application document DE 10 2008 001 121 A1, and the observer-based method in published German patent application document DE 10 2008 040 737 A1.
Published German patent application document DE 10 2008 040 737 A1 discloses a method for monitoring dynamic properties of a broadband lambda probe, in which a measured lambda signal that corresponds to an oxygen concentration in the exhaust gas of an internal combustion engine is determined by way of the broadband lambda probe, in which the internal combustion engine has associated with it an observer that generates a modeled lambda signal from input variables, and in which from the difference between the modeled lambda signal and the measured lambda signal, or from the difference between a signal derived from the modeled lambda signal and a signal derived from the measured lambda signal, an estimation error signal is created as input variable of a controller in the observer upstream from a model. Provision is made here that an indication of the dynamic properties, characterized by a dead time and reaction time, of the broadband lambda probe is determined from an evaluation of the estimation error signal or of a variable derived therefrom, and that the indication of the dynamic properties is compared with defined limit values in order to assess the extent to which the dynamic properties of the broadband lambda probe are sufficient for an intended operating mode of the internal combustion engine.
Published German patent application document DE 10 2008 001 569 A1 furthermore describes a method and an apparatus for online adaptation of an LSU dynamic model. The document relates concretely to a method and an apparatus for adapting a dynamic model of an exhaust gas probe which is a constituent of an exhaust gas duct of an internal combustion engine and with which a lambda value for regulating an air/fuel composition is determined, a simulated lambda value being calculated in a control device and in a diagnosis device of the internal combustion engine concurrently therewith, and both the simulated and the measured lambda value being utilized by a user function. Provision is made here that during vehicle operation, a step behavior of the exhaust gas probe is determined by evaluating a signal change upon excitation of the system, and on the basis of these results the dynamics model of the exhaust gas probe is adapted.
Known functions for monitoring the dynamics of broadband lambda probes are employed for identification of the sensor properties. The requirements for other gas concentration signals of exhaust gas sensors, e.g. for an NOx signal, are comparable to those for O2 signals and O2 sensors. It is therefore to be assumed that there are similarities between the monitoring functions.
The method according to published German patent application document DE 10 2008 001 121 A1 involves active monitoring. It contains an excitation by way of a test injection, which increases not only fuel consumption but also emissions. The method according to published German patent application document DE 10 2008 040 737 A1 operates passively, but requires a so-called “observer” that is complex in terms of application. In addition, both methods are directed primarily toward the detection of larger changes in dead time.
A known method for detecting dynamics uses step-like adjustments to the air/fuel ratio, on the basis of which the dynamics of the probe are evaluated as a function of direction by calculating the ratio of the areas under the step response of the measured air/fuel ratio and of a simulated one. No detection or differentiation of time-constant errors and/or dead-time errors is possible; the procedure is entirely heuristic.
A first-order filter having a time constant T and a gain K=1, as well as a dead-time model having a dead time Tt, are used to model the air/fuel ratio in the control unit. The first-order filter can accordingly be described as follows:G(s)=Kexp(−Tts)/(Ts+1)  (1).
In order to reduce the negative effects of an asymmetrical time constant and/or dead time, for example an oscillating control system, when the time constant or dead time is known to be asymmetrical the measured air/fuel ratio is symmetrized in the control unit using a so-called “symmetrization” filter. For this, the undelayed and/or filtered edge of the signal is artificially delayed with an additional dead time and/or filtered with an additional filter in the control unit; the dead time and/or time constant used corresponds to the diagnosed asymmetrical dead time T+t and/or time constant T+, and the direction of the signal (rich to lean or lean to rich) is determined on the basis of a filtered derivative of the measured lambda signal.
For a system having a slowed-down probe, it is therefore assumed that the nominal model G(s) is supplemented with a further first-order filter as well as a dead-time model:G+(s)=G(s)K+exp(−T+ts)/(T+s+1)  (2).
Once the symmetrization filter has been applied, the entire signal (rich to lean and lean to rich) is symmetrically delayed using two dead times and/or two time constants. This additional delay can be accounted for in the controller by adapting the controller to the greater dead times and/or time constants while retaining its structure, or the increase in the order of the model can even be taken into account by increasing the order of the controller.
A further method, which is known from an as yet unpublished application document of the Applicant, likewise uses a step-like adjustment of the air/fuel ratio but evaluates the slope of the step response and explicitly calculates therefrom a dead time and time constant.
Also known from the literature (Isermann: “Identifikation dynamischer Systeme” [Identification of dynamic systems], Vols. 1 and 2; Nelles: “System Identification”; Ljung: “System Identification—Theory for the User”) are so-called online identification methods with which dead times, time constants, and gain factors, or in general the parameters of dynamic systems, can be determined during normal driving operation. A prerequisite for this is ongoing excitation of the system, on the basis of which the online identification system determines the relevant dead times using recursive optimization methods.
These methods take into account only symmetrical dead times and time constants, however. High-pass filters can be used in this context to suppress offsets or other low-frequency interference signals (Isermann: “Identifikation dynamischer Systeme, Vol. 2) so that the offset does not need to be explicitly estimated.
An online identification method for asymmetrical dead times and time constants is described in an as yet unpublished application document of the Applicant which is based on a so-called “symmetrization” filter. Methods that determine direction-dependent dynamic parameters such as time constants, but not dead times, are also known from the literature.
An as yet unpublished parallel application of the Applicant describes a method for the identification of asymmetrical dead times which is based on cross-correlation or cross-energy, and on the use of high-pass-filtered signals in combination with saturation characteristic curves.
In order to allow asymmetrical time constants also to be identified, this method can be combined with a method for the identification of time constants based on signal energy, which method is described in a further parallel application of the Applicant; the results of these two methods depend on one another, since a time-constant error can also be interpreted as a dead-time error and vice versa. The influence of gain also remains unaccounted for, so that a gain error influences the identification of the time constants. These methods moreover operate iteratively, so that either multiple measurements are necessary or the measured values must be buffered.
All the methods can work both with the air/fuel ratio and with the inverse air/fuel ratio.
In order to improve and enhance the robustness of dynamics monitoring for gas sensors, in particular exhaust gas probes, which can be designed as continuous lambda probes, the object of the invention is to make available a corresponding method that on the one hand operates continuously and on the other hand identifies, in particular, asymmetrical parameters of these dynamic systems.
A further object of the invention is to make available a corresponding apparatus for carrying out the method.